1. Field of the Invention
The present invention relates to a method for the production of half-tone printing forms where the engraving is accomplished with a plurality of small cups arranged in a printing screen and the engraving means is supplied an image signal and a screen signal to produce a printing screen and the engraving needle of the engraving means receives a periodically re-occurring amplitude value of the screen signal to produce its largest deflection in the direction of the printing form and wherein the analog image signal obtained by pattern scanning is digitalized and stored in an intermediate storage with the frequency of the screen signal into image values of predetermined word lengths which is superimposed after a subsequent digital analog conversion for driving the engraving needle and wherein during the pattern scanning, a recognition signal are generated to recognize sharp changes in tone density in the pattern.
2. Description of the Prior Art
U.S. Pat. No. 4,013,829 discloses an engraving machine for producing printing forms from patterns. The patterns may be continuous-tone patterns, so-called line patterns with writings and line representations or combinations of both elements.
The pattern to be reproduced is attached to a rotary scanning cylinder and is scanned by a scanning means which is moved along parallel to the scanning cylinder. Depending upon the tonal values of the scanned image points, more or less light will be reflected from the pattern into the scanning means and there converted electro-optically into an image signal which is digitalized by means of a scanning cycle sequence.
For the purpose of an electronic equalization of different diameters of the scanning and printing cylinders in the reproduction, the digital image signal is recorded with the aid of a scanning timing sequence and placed in an intermediate memory where it is again read out by means of a recording timing sequence and reconverted into an analog image signal. The analog image signal is fed to an electromagnetic engraving means which has an engraving pin as a cutting tool which moves axially along a rotating printing cylinder.
During engraving of the printing form, the engraving means engraves a sequence of cups arranged in a seamless manner in a printing screen with the depth depending in each case on the tonal value of the associated image dot.
For the screening, the analog image signal is superimposed on a screen signal which has a frequency that depends on the printing screen to be recorded and the surface speed of the printing cylinder.
The screen signal causes a vibrating lifting motion of the engraving pin corresponding to the printing screen and determines the instances when the engraving of the cups occur and the momentary image signal determines the depth of penetration of the engraving pin into the cup at the particular instant.
The engraved printing cylinder forms the printing form for the later printing process in a photogravure rotary machine.
The exact reproduction of contours in the pattern represents a particular problem in the production of screened printing forms. In the engraving system described in U.S. Pat. No. 4,013,829 the positions of the cups on the printing form are predetermined by the printing screen. On the other hand, during scanning, the contour lines extend arbitraily with regard to the scanning screen which is congruent with the printing screen to be recorded. Fine charges of edges therefore can be reproduced only in an unsatisfactory manner with a rigid cup arrangement. For example, fine edges extending obliquely to the scanning direction show a disturbing staircase-like arrangement.
Additionally, in prior art engraving systems, the shade values in the range of an edge are reproduced in a varied manner because of the transfer characteristics because of the mass of the electro-mechanical engraving means which results in a time delay between the time that the signal is applied and the actual physical movement of the cutting tool.
It is known that a sharper edge reproduction could be accomplished by using a finer printing screen. However, a finer printing screen has the drawback that during subsequent printing process the transfer of ink from the cups to the printing form is defective due to the smaller cup volumes of the finer printing screen.
U.S. Pat. No. 3,983,319 comprises an improvement for forming the edges by position shifting of screen points within the area of the edge.
In the reproduction apparatus for the exposure of screen images upon a photosensitive medium as described in U.S. Pat. No. 3,983,319 for the purpose of a sharper reproduction of the edge those screen points which reproduce edges are so shifted with respect to the positions predetermined by the printing screen that they subsequently impinge upon the edges.
In this reproduction apparatus, patterns are scanned additionally with a photo diode array arranged transversely to the scanning direction. With the presence of an edge extending in the scanning direction in the pattern, this photo diode array produces a control signal for the recording apparatus. The control signal moves a rotary mirror located in the path of rays of the luminous source for the recording such that the screen points are exposed and shifted perpendicularly to the direction of recording.
This reproduction system has the disadvantage that only edges extending in the scanning direction can be improved in the reproduction. A precise determination of the location of an edge is not possible.
Another disadvantage of this apparatus is that the method is not applicable for electro-magnetic engraving means because the engraving needle cannot be deflected rapidly either in the recording direction or transversely thereto but merely performs a stroke movement directly radially relative to the printing form.
German Publication OS No. 2,516,332 discloses that a reduction in the amount of data for reproduction of text can be utilized because a lower number of density steps is adequate for texts compared to reproduction of continuous-tone patterns.
In the method disclosed in the prior art a difference is made therefore betwen a continuous-tone code and a shorter text code. In the event that the pattern has continuous-tone and text areas the scanned density values are first and simultaneously converted into the continuous-tone code and into the text code. The selection of a code then is made by means of a control signal which is derived by using mask scanning or from measuring the surface extend of the individual areas of the pattern.
This known prior art method does not reduce the required memory capacity. Because the portion of continuous-tone and text areas in a pattern fluctuate, the memory must always be large enough to accommodate the largest quantity of data which occurs in a pure continuous-tone pattern.
The method of the prior art also has the disadvantages in that a control mask must be made for each pattern which is extremely time consuming and expensive. Additionally, the extensions of the pattern areas must be known so that the method is not universally applicable. It is also a disadvantage that the control signal is generated in each case only as a function of the pattern areas while the density leaps or jumps are not considered in a continuous-tone range.